Do meteorites contain the building blocks of life?
Some do. Carbonaceous chondrites contain amino acids, the components of DNA and RNA called nucleobases, and even sugars, all made by natural chemistry in space. They contain no life, but they show that life's ingredients formed widely.
Life on Earth is built from a handful of molecular families: amino acids, which link up to make proteins; nucleobases, the letters of the genetic code; sugars; and fatty compounds that form cell membranes. Remarkably, versions of all of these have been found in meteorites.
The classic example is Murchison, a carbonaceous chondrite that fell in Victoria, Australia, in 1969. Because the fall was recovered quickly and the space programme had just developed clean techniques for studying lunar samples, scientists could analyse it carefully. They found dozens of amino acids, some of which are rare or absent in Earth's biology, alongside a huge variety of other organic compounds. Later studies of Murchison and similar meteorites reported nucleobases, including all five used in DNA and RNA, and the sugar ribose, which forms the backbone of RNA.
How can scientists be sure these molecules are not contamination from Earth? Several clues help. Some amino acids in meteorites are unknown in terrestrial life. The carbon, nitrogen and hydrogen often contain unusual proportions of heavier isotopes, a chemical fingerprint of formation in cold space. And amino acids appear in left- and right-handed forms in near-equal amounts, whereas life on Earth uses almost only the left-handed kind.
Asteroid samples returned by spacecraft make the case even stronger, because they never touched Earth's surface. Material from asteroid Bennu, delivered in 2023, contains 14 of the 20 amino acids used in Earth's proteins and all five nucleobases. None of this means meteorites carried life. It shows that the chemistry leading towards life happens naturally in space, and that the young Earth was probably sprinkled with these ingredients.